Liquid developing agents in use are generally of the type comprising electrically insulating liquid in which colored resin particles containing pigment or colorant (hereinafter referred to as “toner particles”) are dispersed. Various methods are used to manufacture such liquid developing agent, including: (1) the polymerization method (monomer components are polymerized in an electrically insulating liquid in which colorant is dispersed, in order to form colored resin particles), (2) the wet pulverization method (colorant and resin are mixed and kneaded at or above the melting point of the resin, after which the mixture is dried and pulverized and the resulting pulverized matters are wet-pulverized in an electrically insulating liquid in the presence of dispersant), and (3) the separation method (hereinafter referred to as “coacervation method”) (a liquid mixture comprising colorant, resin, solvent that dissolves the resin, and electrically insulating liquid that does not dissolve the resin, is prepared and then the solvent is removed to separate the resin, after which colored resin particles are dispersed in the electrically insulating liquid).
The coacervation method, which is one method of manufacturing such liquid developing agent, is explained in connection with the present invention. Under the coacervation method, a liquid mixture comprising solvent that dissolves resin and electrically insulating liquid that does not dissolve resin is prepared, and then the solvent is removed to cause the dissolved resin in the liquid mixture to separate as colored resin particles in which colorant is encapsulated, and the colored resin particles are also dispersed in the electrically insulating liquid.
Liquid developing agents obtained from such method are considered to have good electrophoretic property because their colored resin particles are closer to a spherical shape than those in liquid developing agents obtained by the wet pulverization method, and the particles are also uniform in size.
However, images printed with a liquid developing agent obtained by the coacervation method present problems in that they have lower abrasion resistance than those printed with a liquid developing agent obtained by other methods.
Methods that have heretofore been proposed to improve the abrasion resistance of liquid developing agent include methods wherein wax is added (refer to Patent Literature 1, for example), and methods wherein toner particle composition resin having a specific melting temperature is used in combination with additive material (polyethylene wax, etc.) (refer to Patent Literature 2, for example).
In light of the above, the applicant for the present application for patent proposed a method wherein polyethylene wax, etc., as proposed above is contained in order to improve the abrasion resistance of liquid developing agent obtained by the coacervation method (refer to Patent Literature 3). Images obtained by this method present somewhat improved abrasion resistance, but the level is not sufficient, making this a lingering problem specific to the manufacture of liquid developing agent using the coacervation method.